Screw pumps



Nov. 13, 1962 c. o. T. MONTELIUS I 3,063,379

SCREW PUMPS Filed Feb. 23, 1960 FIG. I.

Z G F INVENTOR.

CARL OSCAR TORSTEN MONTELIUS' ATTORNEYS United States Patent SCREW PUMPSCarl Oscar Torsten Montelius, Stockholm, Sweden, as-

signor to De Laval Steam Turbine Company, Trenton,

N.J., a corporation of New Jersey Filed Feb. 23, 1960, Ser. No. 10,398Claims priority, application Sweden Feb. 23, 1959 1 Claim. (Cl. 103-128)The present invention relates to an improvement in screw pumps of thetype which comprises an assemblage of screws including a middle screwwith convex thread flanks and one or more side-screws meshing with themiddle screw and having concave thread flanks, the threads being of suchconfiguration that they seal against the circumference of cooperatingscrews, all said screws being sealingly enclosed in a casing formed withintersecting bores for the screws.

In the operation of such a pump, the liquid being pumped is progressedaxially from the inlet end or suction side of the screw assemblage tothe outlet end or pressure side thereof in essentially closed chambersformed by the thread flanks and bottoms of the screws and the enclosingcasing. Such a chamber is formed at the suction side of the screwassemblage when thescrews are rotated and is filled with liquid while itis formed. When the chamber is fully formed, it is closed toward thesuction side and on continued rotation of the screws it travels axiallyalong the screw assemblage toward the outlet end of the screw assemblagewhere it is opened and the liquid is discharged. The screw assemblagemay be such that a chamber starts to open toward the outlet end as soonas it is closed toward the inlet end. This can be considered as a limitcase whereinthe're is only momentarily a fully closed chamber betweenthe inlet end and the outlet end. In such a ca'se,ho'wever, the inletend and the outlet end are alwaysseparated' by a closure or seal formedby the screw threads. The volume of the chamber is unchanged while itprogresses from the inlet to the outlet end, and provided that. the pumpis ideally tight, the liquid in the chamber is throughout this travelsubjected 'to the pressure prevailing at the inlet end to be subjectedto the higher pressure prevailing at the outlet end only when thechamber is opened at the outlet end. In an actual pump of this type,-ofcourse, a certain amount of leakage will occur on account of unavoidabletolerances in the manufacture which result in a certain amount of playas between the screws and as between each screw and the casing,wherefore a small increase in the pressureon the liquid will occurduring travel along the screw assemblage. With a sufiicient accuracy inmanufacture, however, this increase in pressure will be so small thatsubstantially the entire increase in pressure occurs'at the outlet end.

If the screws and the casing have a certain minimum length, the abovelimit case will be obtained, wherein there is constantly a single sealbetween the inlet end and the outlet end.

This minimumf-length is determined in the following manner: The minimumlength (L of the screws to obtain only one seal between the suction sideand the pressure side is the highest of the values determined by the twoequations and the minimum length of the casing is determined by theequation ICC In these equations:

i is the number of threads of the middle screw i is the number ofthreads of the side screws 6 is the thread top angle of the middle screw(in degrees) S is the thread lead of the middle screw is the half angleof intersection (in degrees) of the bores of the casing as seen from thecenter of the middle screw.

It is of interest to note that when these equations are used, the lengthof the screws will not be equal to the length of the casing. Thedifference between the two lengths will be equally apportioned to theends of the screw assemblage. The length of the screws in thisconnection refers to the efiective length of the threads, i.e. thelength in which they mesh with each other. The length of the casingrefers to the portion thereof which sealingly surrounds the screws.

If the pump is to operate at a higher pressure, it is necessary toincrease the lengths of the screws and the casing, so that there aremore seals between the inlet and outlet ends. In such cases, the lengthscan be selected so that the number of seals is constant during theoperation of the screws. This can be achieved in diiferent ways.

It the number of seals in each thread groove is to be constant, thelengths of the casing and the screws will be increased by an amountequal to AL=(i,1)Z:-S 1v where i, is the number of seals desired. (Here,as well as in connection with Equations V and VI below, the expressionthread groove relates to the thread grooves of the screw having thegreatest number of threads.) Under certain circumstances, however, itmay be advantageous to permit difierent numbers of seals between theinlet and the outlet end in the various thread grooves, the sum of thenumber of seals in the various thread grooves being constant for allangular positions of'the screws. The amount by which the casing andscrew lengths will be increased will then be equal to wherein i is thegreatest number of seals in any thread groove in any position of thescrews m is the number of thread grooves having i seals in such position1' is the greatest of i and i In this case the total number of sealswill be m+( 1zm1) V1 If the lengths of the casing and the screws areselected in other ways, so that the above equations are not satisfied,then a varying number of seals will be obtained in different angularpositions of the screws. Such lengths may be advantageous from otherpoints of view, e.g. to achieve sufficiently large bearing surfaces ofthe screws. The seals between the pressure side and the suction side areof diflerent types. There are seals between the peripheral surfaces ofthe screws and the casing, and there are seals between the peripheralsurfaces and cores of the screws and between the peripheral surfaces andthread flanks. However, as mentioned above when there is more than oneseal, in cases where for some reason or another the screw length isselected so that the above Equations IV, V and VI are not satisfied, thenumber of seals varies with the angular positions of the screws duringeach revolution of the middle screw. As above mentioned, on account ofunavoidable plays the seals cannot be made perfect in practise, but aninternal leakage must always occur. When the number of seals between thepressure side and the suction side varies during each revolution, thismeans that the internal leakage will also vary during each revolution.Since a positive pump of this type gives a capacity which is equal to a.theoretical quantity per revolution reduced by the internal leakage, itwill be understood that the rate of flow through the pump will fluctuateor pulsate slightly due to this variation of the internal leakage. It isdesirable, however, to obtain a fiow which is free'from pulsation.

The object of the present invention is to provide a screw pump of thetype referred to which has a nonpulsating flow by keeping the number ofseals constant for each revolution. It has been found that this can beachieved by providing a shallow circumferential groove in any thread ofthe middle screw extending from one flank to the other of such thread insuch a manner that one seal between the suction side and the pressureside is destroyed in certain angular positions of the revolution of thescrew.

The invention will be described more in detail with reference to theaccompanying drawings, wherein:

FIGURE 1 shows an embodiment of the invention in elevation, partly insection;

FIGURE 2 shows a cross-section taken along line II--II in FIGURE 1.

The pump illustrated on the drawings is of the type comprising a drivenmiddle screw 1 and two side-screws 2, 3, the middle screw 1 having twothreads with convex flanks and the side screws also having two threadseach but with concave flanks and of oppposite hand to the threads of themiddle screw, the threads being formed in a manner known per se so as tobe in sealing relationship with each other. The screw assemblagecomprising the middle screw 1 and the side-screws 2, 3 is enclosed in acasing 4 which sealingly surrounds the screw assemblage, said casinghaving at its lower end two symmetrical ports 5 through which theentrance of fluid to the screw assemblage takes place, the fluid beingdischarged through the open top end of the casing. The casing 4 has anattachment flange 6 for attaching it by means of screws 7 in a pumphousing 8 so that the lower end of the casing 4 with the ports 5 isdisposed in the inlet chamber 9 of the pump housing, and the upper endis disposed in the discharge chamber 10 of the pump housing.

The middle screw 1 is driven by a motor (not shown) through a drivingshaft 11 which is journaled in a bearing 12 attached to the top end ofthe casing 4 and extends through a' cover -13 attached to the pumphousing.

As best seen in FIGURE 2, the casing is formed with a larger centralbore for the middle screw 1 and two smaller bores for the side-screws 2,3, each of the two latter bores intersecting with the first bore.

The screw pump shown in FIGURES 1 and 2 represents a type which iscommon in practice and is dimensioned so that the effective length ofthe casing as well as the effective length of the screws (the distance Lin FIGURE 1) is equal to 3D, where D is the external diameter of themiddle screw. This represents only an example and is not intended tolimit the invention. In a screw assemblage formed in this manner it willbe found that during a part of each revolution of the middle screw thereis one seal between the suction side and the pressure side, while duringanother part there are two seals. The leakage will therefore vary duringeach revolution so that a pulsating flow is obtained. However, bydestroying one seal, in accordance with the present invention, so thatthere is only one seal during each revolution, the leakage is equalizedresulting in a nonpulsating flow from the pump. This is. achieved inaccordance with this invention by providing a groove 14 in the threadcircumference of the middle screw.

The groove 14 may be disposed at either end of (the threaded portion of)the middle screw, but this gives substantially the same result as if thelength of the screw had been selected according to the above formula forobtaining a constant number of seals. It is more advantageous to providethe groove intermediate the ends of the threaded portion of the screw,so as to obtain satisfactory bearing surfaces on the middle screw atopposite sides of the groove. The distance (x in FIGURE 1) from one endof the middle screw shall be equal to the minimum length L for obtainingconstantly one seal, as determined by Equation I or II above. Thedistance x may be taken from either end of the screws.

If the groove is disposed at either end of the screw, the width (W)thereof will-be equal to where L the actual effective length of thescrews L ==the minimum length of the screws to obtain one seal(according to Equation I or II) n=a positive integer selected so that OWS /i The case, where n can be selected so that W is equal to 0 occurswhen the length of the screws is such as to correspond to a constantnumber of seals in which case no groove is necessary. This case isincluded above for the sake of completeness to facilitate theunderstanding of the following.

However, when the groove is disposed intermediate the ends of the screw,as is preferred, the width W must be increased by an amount equal to ascompared to W The reason therefor is that in the contacting surfacesbetween the middle and side screws, the threads of the screws engageeach other in an axial direction. Thus, in this case the width will beequal to On account of the axial engagement of the threads with eachother, asmentioned above, it will be necessary, even when the groove isto be disposed intermediate the ends of the screws, to determine whetherW will be 0, in which case the length of the screws is such as tocorrespond to a constant number of seals and the groove will beunnecessary. If W 0, the width of the groove will be determinedaccording to Equation VIII.

For screws having a screw length L such that (in which case Equation VIIabove gives a negative value for W the width can be chosen freely, butshould be chosen as small as possible (although of course not so smallthat the desired function of the groove is not achieved) in order thatthe bearing surfaces shall be as. large as possible.

The angles l// and 0 referred to in connection with the Equations I, IIand III above are indicated in FIGURE 2.

It will be understood that the invention is not restricted to theembodiment shown comprising a driven twothreaded middle screw and twotwo-threaded side-screws, but is generally applicable to otherscrew-pumps having a different number of side-screws and a differentnumber of threads, the shape of the threads and the number ofside-screws as well as the number of threads of the screws being soselected in relation to each other, in a manner known per se, that ineach position of the screws there is at least one seal between the endsof the screw assemblage. It is known to those skilled in the art thatthis can be realized if the threads obtain a certain mathematicallydefined geometrical shape, and if the condition is satisfied, where G isthe number of threads of the middle screw, 12 is the number ofside-screws and g is the number of threads of each side-screw. Theinvention is applicable to all screw pumps of this type, including suchscrew pumps of this type where not only the middle screw but also theside-screws are driven.

I claim:

A positive screw pump of the type comprising a screw assemblageincluding a middle screw with convex thread flanks and at least one sidescrew with concave thread flanks and a casing enclosing said screwassemblage and having intersecting bores for accommodating said screws,said casing providing inlet and outlet passages, the screws sealingagainst each other and against the casing to form constant volume closedchambers within which the pumped fluid is conveyed axially along thescrew assemblage from its inlet passage to its outlet passage, thelength of the screws and a continuously closed length of the bores inthe casing being such that there is always at least one closure in thescrew assemblage, the last arrangement being such that, without thecharacteristics specified hereafter the number of closures would varyduring rotation of the screws, characterized by the fact that any threadof said middle screw is provided with a shallow circumferential grooveat its periphery extending from one flank to the other of said thread,located between the ends of said middle screw and between the ends ofsaid continuously closed length of the bores, which groove has suchaxial extent and is so located as to interrupt the complete closure ofchambers during portions of revolutions of the screws to maintainconstant the number of closures existing throughout each revolution ofthe screws.

References Cited in the file of this patent UNITED STATES PATENTS

